1. Field of the Invention
The present invention relates to a process for preparing microcapsules containing an oily liquid. More particularly, the present invention relates to a process for preparing microcapsules containing a hydrophobic oily liquid by using gelatin as at least one type of hydrophilic colloid in the complex coacervation method. The process is characterized by adding an aqueous solution of an anionic surface active agent in combination with a shock-preventing agent at a temperature lower than the gelling point of gelatin to prevent the increase of viscosity caused by the reaction of gelatin and the hardening agent during the hardening pretreatment process and promoting the hardening pretreatment.
As used herein, the term "hardening pretreatment" means the operation of rapidly converting the pH of the system to the alkaline region to promote the reaction of gelatin and the hardening agent. Typical examples of hardening agents include aldehydes such as formaldehyde, glutaraldehyde etc, glyoxal, methylglyoxal, aldehyde-modified starch, etc. The term "shock" as used herein, describes the phenomenon in which, in carrying out the hardening pretreatment of a coacervate containing an oily liquid, the viscosity is rapidly increased to such a point that the pH of the system is between 6 to 9, i.e., a phenomenon in which the microcapsules deposit on each other resulting in flocculation or coagulation as the viscosity is increased.
2. Description of the Prior Art
Conventionally known processes for preparing oil-containing microcapsules are described, for example, in the specification of U.S. Pat. No. 2,800,457 and Japanese Patent Publication No. 3875/62. The process of U.S. Pat. No. 2,800,457 comprises the steps of (1) emulsifying a water-immiscible oil in an aqueous solution of a first hydrophilic colloid to be ionized in water (the first sol), the emulsifying step, (2) admixing an aqueous solution of a hydrophilic colloid in the emulsion of (1) forming the second sol, the first sol having an electric charge opposite to that of the second sol, and either adding water thereto or adjusting the pH thereof to cause coacervation, thus obtaining coacervates wherein the complex colloid is fixed to (i.e., surrounds) the individual oil drops (the coacervation step), (3) cooling the coacervates to cause gelation thereof (the gelation step), and (4) adjusting the pH to from 9 to 11 and adding a hardening agent (the hardening treatment). However, although the specification states that, according to this process, the size of microcapsule decreases as the amount of water used is decreased, mononuclear microcapsules cannot be obtained. Thus, the control of the size of the microcapsule is limited to from 20 to 30 microns when the oil droplet size is within a range of from 5 to 6 microns. The process of the Japanese Patent Publication Gazette No. 3875/1962 is a process in which a thickener such as Acacia, Tragacanth, methylcellulose, carboxymethyl cellulose, magnesium aluminum silicate, polyglycol, glycerin, syrup, and the like, is added to accumulate a hydrophilic colloid surrounding the oil droplets in step (1) of the above described U.S. patent, i.e., the emulsifying step. According to this process, the coacervation can only be affected in a higher colloid concentration than that in the above mentioned U.S. patent even where the surface characteristics of the emulsified droplets are changed by coexistence with the thickener before the coacervation step. However, mononuclear microcapsules cannot be prepared. This is due to the fact that the particle size of the microcapsules produced have already been determined according to the condition of coacervation established and, even if an unsatisfactory coacervation system is established, the microcapsules are coagulated (shocked) in the hardening pretreatment step in hardening the coacervate wall.
Microcapsules containing the oily liquid, prepared according to either of these processes are multinuclear microcapsules in the form of a "grape" bunch, (as used herein the term "mononuclear" refers to a single emulsified oil droplet and the term "multinuclear" refers to aggregated emulsified oil droplets.) Such multinuclear microcapsules have the disadvantage that, when used in a pressure-sensitive copying paper utilizing the reaction of a leuco dye and a clay, the resolving power of the colored letters in copying using a plurality of sheets is deteriorated because of the large particles. When coating on a paper by means of an air knife, microcapsules prepared by these processes are subject to a classification action whereby the composition of the liquid is changed. Therefore the air pressure used must be elevated. Thus, an increase in coating speed is obstructed.
An object of the present invention is to provide a process for preparing mononuclear and/or multinuclear (especially mononuclear) microcapsules containing an oily liquid having a great resistance.
Another object of the present invention is to provide a process for preparing microcapsules containing an oily liquid, whereby an increase in viscosity during conversion of the pH to alkalinity is prevented, a rapid conversion of the pH is carried out, the coating speed is increased and the drying step is shortened.